1. Field of the Invention
This invention pertains generally non-invasively probing the inner structure of the skin of a patient, and more particularly to probing the inner structure of the skin of a patient through time-resolved fluorescence.
2. Description of Related Art
Fluorescence is the physical phenomenon in which light is emitted by a substance as a result of excited electrons returning to their ground states after absorption of excitation light. Substances that emit fluorescence (fluorophores) are characterized by their quantum yield, their fluorescence lifetime(s), and their emission wavelengths. Emission takes place over a spectral range and at wavelengths larger than the excitation wavelength. The quantum yield is the ratio of the number of photons emitted to the number absorbed while the fluorescence lifetime is the average time the electrons spend in their excited states. Biological tissues contain several endogenous fluorophores such as NADH, aromatic amino acids like tryptophan and structural proteins such as collagen and elastin. The optical properties of these fluorophores are sensitive to the environment and the metabolic status of the tissue making fluorescence spectroscopy a valuable tool to study the health of biological tissues.
Practically, fluorescence spectroscopy techniques consist of exposing the medium or tissue of interest to excitation light (typically UV) and measuring the fluorescence emission spectrum. The incident excitation can be a continuous or an ultra-short pulse beam of light. It can also be collimated or diffuse whether a laser or an arc-lamp is used. These measurements can be carried out in (i) a monochromatic or spectral and (ii) steady-state or time-resolved manner. Spectral measurements typically involve either emission spectral measurements or excitation spectra measurements. Fluorescence emission spectrum measurements consist of measuring the fluorescence intensity over a range of wavelengths for affixed excitation wavelength. On the contrary, excitation spectra measurements consist of measuring the fluorescence intensity at a particular wavelength for a range of excitation wavelengths.
Fluorescence of skin has been proposed as a means of diagnosing pathologic tissue by analysis of observed fluorescence spectra. Researchers have shown the feasibility of using the fluorescence spectra of skin as a means of detection of cancer of the skin. It has also been demonstrated that fluorescence can be used to distinguish fibrous plaque from healthy arterial wall. (Sterenborg H. J. C. M., Motamedi M, Wagner R. F., Thomsen S. L., Jacques S. L., 1994, In Vivo fluorescence Spectroscopy for the diagnosis of skin diseases, “SPIE Proceedings of Optical Biopsy and Fluorescence Spectroscopy and Imaging, edited by R. Cubeddu, R. Marchesini S. pp. 32-38.)
Recently, time-resolved fluorescence measurements have been used to identify malignant tissue (Das B. B., Feng L. and Alfano R. R, 1997, Time-resolved fluorescence and photon migration studies in biomedical and model random media, Reports on Progress in Physics, Vol. 60, No. 2, pp. 227-292).
They showed that time resolved fluorescence measurements can be used to distinguish malignant tumors from non malignant breast tumors. Researchers have recently shown that fluorescence lifetime measurements are generally more robust to scattering artifacts than are measurements of fluorescence spectra, even though they are sensitive to the source detector separation.
Very recently, a steady-state autofluorescence reading device was developed for assessing the accumulation of advanced glycation end products in skin (Meerwaldt, R., R. Graaff, P. H. N. Oomen, T. P. Links, J. J. Jager, N. L. Alderson, S. R. Thorpe, J. W. Baynes, R. 0. B. Gans, A. J. Smit1, 2004. Simple non-invasive assessment of advanced glycation endproduct, accumulation, Diabetologia, Vol. 47, pp. 1324-1330). The fluorescence signal was found to correlate with the presence of several key AGEs in the skin, as well as with diabetes duration, mean HbA1C of the previous year, and creatinine levels. However, the vast majority of the human subjects were Caucasian, and measurements were performed only on the patient's forearm. Moreover, steady-state fluorescence techniques of the above device have several disadvantages that limit their effectiveness: 1) they cannot distinguish fluorophores emitting at similar wavelengths; 2) they are influenced by endogeneous chromophores, which interact with the excitation and fluorescent light; and 3) the fluorescence signal depends on the geometry and the probe design, and the properties of the skin such as pigmentation.
Various methods have been developed to simulate transient radiation transport in absorbing and scattering media. The Monte-Carlo method is often used to simulate such problems because of its simplicity, the ease by which it can be applied to arbitrary configurations and its ability to capture real physical conditions. However, it has inherent statistical errors due to its stochastic nature. It is also computationally time consuming and demands a lot of computer memory as the histories of the photons have to be stored at every instant of time. Thus, the Monte-Carlo method is ruled out in practical utilizations such as real time clinical diagnostics where computational efficiency and accuracy are major concerns.
The backward or reverse Monte Carlo has been developed as an alternative approach when solutions are needed at particular locations and times. The method is similar to the traditional Monte Carlo method, except that the photons are tracked from the detector back to the source rather than from the source to the detector as in the conventional Monte Carlo method.
There is no need to keep track of photons which do not reach the detector and so the reverse Monte Carlo method is much faster than the traditional Monte Carlo method. The method was successfully applied by (Lu X. and Hsu P.-F., Reverse Monte Carlo method for transient radiative transfer in participating media, Proceedings of the ASME International Mechanical Engineering Congress and Exposition, 2003) to simulate transient radiative transport in a non-emitting, absorbing, and anisotropically scattering one-UC dimensional slab subjected to ultra-short light pulse irradiation. But again, it has the same disadvantages as the Monte Carlo method: (i) it carries statistical errors and (ii) to solve coupled problems, it is hard to couple with other numerical schemes such as finite volume methods.
The diffusion approximation has been extensively used in biomedical applications in order to simplify light transport in biological tissues as it is simpler to solve than Equation (1). However, its validity for transient light transport in highly scattering media such as biological tissues has been questioned. Indeed, (Elaloufi, R., Carminati, R., and Greffet, J. J., 2002. Time-dependent transport through scattering media: from radiative transfer to diffusion, Journal of Optics A: Pure and Applied Optics, vol. 4, no. 5, pp. S103-S108) have shown that the diffusion approximation fails to describe both short-time and long-time radiation transport in thin slabs for both weakly and strongly absorbing cases. In the case of thick slabs, the diffusion approximation fails for short times. The authors have also shown that the diffusion theory always fails to predict the long-time behavior of transmitted pulses in thin slabs whose optical depth defined by τL=σS(1-g)L is less than eight. Researchers have examined various transport models for the simulation of ultra short laser pulses in turbide media (Kumar S. and Mitra K., 1999, Development and Comparison of Models for Light-Pulse Transport Through Scattering Absorbing Media, Applied Optics, Vol. 38 No. 1 pp. 188-196). They showed that there is a large difference in the temporal shape of the pulses based on the model chosen. Thus model selection would play a great role if experimentally measured temporal signals are used obtain the transport properties of the media.
Accordingly, an object of the present invention is to provide a time-resolved photometric device and the associated analysis software for detection and diagnoses of various medical conditions in a non-invasive, reliable, cheap, and convenient manner.